In the last article we discussed about the cavitation in centrifugal pumps and methods to prevent it. We understood that cavitation is situation that requires serious thought. To avoid this penetrating problem, one must ensure that the pressure of the fluid flowing through the centrifugal pump at all points inside the pump remains above the saturation pressure. The quantity that is used to determine whether the pressure of the fluid that is being pumped into the pump is adequate for avoiding cavitation is termed as NPSH: Net Positive Suction Head.
So before we discuss about NPSH of a centrifugal pump, it is important to know the fundamental working principles of this machine. A centrifugal pump is basically a machine responsible for imparting energy to a fluid for two purposes:
- for increasing the pressure of the fluid or
- moving it along the pipeline.
A centrifugal pump normally accomplishes this task through the actions of blades. As fluid enters the vane of the centrifugal pump, energy is supplied in the form of velocity. When the velocity is gradually reduced, the energy gets converted into pressure. It is clear that a pump will not be able to render velocity to a liquid that is outside the pump. Therefore the liquid must enter from the opening of the impeller and pass through the vanes before the process of transfer of energy begins.
While the conditions outside the centrifugal pump to force the liquid into the eye of the impeller, there must be sufficient energy available to the liquid at the eye to perform this energy transformation process and assure that the liquid remains in a liquid state. This energy is known as the NPSH(A): Net Positive Suction Head Available.
The formula of Net Positive Suction Head Available is the difference between:
the pressure at the suction of the pump and the saturation pressure of the fluid that is being pumped.
NPSH(A) = P(suction) – P(saturation)
When these pumps are designed, they have inbuilt physical and hydraulic characteristics that determine:
- the amount of energy that will be required to push the liquid into the impeller,
- energy needed to ensure that the liquid remains a liquid throughout its path through the impeller.
Some of the factors that also help determine the amount of energy needed by the pump are:
- the nature of the pump eye,
- the structure of the blades of the impeller,
- their diameter,
- the speed at which they operate
The amount of energy needed by a centrifugal pump is called NPSH (R): Net Positive Suction Head Required.
The pump can only perform its functions properly when the energy available is equal to or greater than the energy required. Therefore, one basic rule that every centrifugal pump must follow is:
NPSH(A) ≥ NPSH(R)
And if we consider a margin of safety, then this rule can be best stated as:
NPSH(A) > NPSH(R)
To avoid cavitation, the minimum net positive suction head necessary is known as the Net Positive Suction Head Required.
You don’t want to install a noisy, slow and damaged centrifugal pump for your industry. Hence it is important to get the NPSH(R) value from your pump manufacturer and insure that your NPSH(A) will be enough to cover those needs.